In comparison to the existing type of antenna including a waveguide, which has a plurality of radiating slots arranged in a longitudinal direction thereof, and a radiating horn attached to the waveguide, a slot array antenna including a radiation waveguide, which has a plurality of radiating slots arranged in lateral and longitudinal directions thereof on a two-dimensional radiation plane, has been presented in recent years for the purpose of ease in manufacturing and size reduction (WO2008/018481). The slot array antenna disclosed in WO2008/018481 includes a radiation waveguide arranged with a two-dimensional slot array and a feeding waveguide arranged with a slot array and for guiding (feeding) an electromagnetic wave to the radiation waveguide from a direction orthogonal to a propagation direction of the radiation waveguide for electromagnetic wave, which are coupled with each other (see WO2008/018481).
The feeding waveguide coupled to the radiation waveguide disclosed in WO2008/018481 has in general, structures shown in FIGS. 25A and 25B. That is, the structure shown in FIG. 25A is what a feeding waveguide 100 is simply coupled to a radiation waveguide 200 in an orthogonal direction (width direction) thereto, and the structure shown in FIG. 25B is what is designed to keep the dimension of a feeding waveguide 101 in the width direction of the radiation waveguide 200 the same as or smaller than the width dimension of the radiation waveguide 200 by bending the feeding waveguide 101 to be an “L” shape.
In the structure shown in FIG. 25A, a section of the feeding waveguide 100 corresponding to feeding slots 100a are contained within the width dimension of the radiation waveguide 200; however, a section of the feeding waveguide 100 on a base end 100b side protrudes outside the width dimension of the radiation waveguide 200, thus limiting size reduction of the slot array antenna. Further, in the structure shown in FIG. 25B, although the structure of the feeding waveguide 101 is contained within the width dimension of the radiation waveguide 200, feed characteristics of an electromagnetic wave from feeding slots 101a of the feeding waveguide 101 to the radiation waveguide 200 become ununiform, particularly in width directions, due to a discontinuous section such as a bent section 101c of the feeding waveguide 101. As a result, a transmission mode pattern of the electromagnetic wave which propagates within the radiation waveguide 200 breaks down.